.beta.-substituted-.beta.-propiolactone polymers are made by a wide variety of bacteria to serve as a source of both energy and carbon supply. Perhaps the best known of these bacterially produced polyesters is poly(.beta.-hydroxybutyrate) (PHB) characterized by a structure having a repeat unit of the formula: ##STR1##
The bacterially-produced form of poly(.beta.-hydroxybutyrate) is a highly crystalline, optically active and perfectly isotactic polyester.
Ring-opening polymerization of .beta.-substituted-.beta.-lactones in the presence of organometallic catalysts to produce .beta.-substituted-.beta.-propiolactone polymers is known. The use of alkylaluminum-water based polymerization initiators or catalysts is described by Benvenuti and Lenz in U.S. Pat. No. 5,023,316 and others (for example see Agostini, D. E.; Lando, J. B.; Shelton, J. R. J. POLYM. Sci. PART A-1 1971, 9[A], 2775-2587 and 2789-2799; Gross, R. A.; Zhang, Y.; Konrad, G.; Lenz, R. W. MACROMOLECULES 1988, 21, 2657-2668). Polymerization of .beta.-butyrolactone with aluminum porphyrins is described by Aida, T.; Maekawa, S.A. and Inoue, S. in Macromolecules 1988, 21, 1195-1202. The use of polymerization initiators derived from organotin compounds is described by Kemnitzer, J. E.; McCarthy, S. P.; Gross, R. A. in MACROMOLECULES 1993, 26, 1221-1229 and Hori, Y.; Suzuki, M.; Takahashi, Y.; Yamaguchi, A.; Nishishita, T. in MACROMOLECULES 1993, 26, 5533-5534. The use of organozinc-water based initiators for the polymerization of .beta.-substituted-.beta.-lactones is reported by Zhang, Y.; Gross, R.A.; and Lenz, R.W. in MACROMOLECULES 1990, 23, 3206-32-12 and Tanahashi, N.; Doi, Y. in MACROMOLECULES 1991, 24, 5732-5733. Le Borgne, A. and Spassky, N in POLYMER 1989, 30, 2312-2319 describe the use of a chiral alkylzinc alkoxide initiator for the polymerization of racemic .beta.-butyrolactone. Kumagai, Y. and Doi, Y. in J. ENVIRON. POLYM. DEGRADN. 1993, 1, 81-87 describe the use of a zinc-water-polymeric alcohol initiator for the polymerization of this same monomer.
A disadvantage of each of the above systems mentioned for the polymerization of .beta.-substituted-.beta.-propiolactones to polyesters is that each system requires either long reaction times, production of low yields of polymer, limited to lower molecular weight polyester products, broad molecular weight distributions, production of mixtures of isotactic, atactic, and/or syndictactic polymer, or combinations of these deficiencies.
The alkylaluminum-water systems cited above are commonly referred to as "aluminoxane" catalysts or initiators. The yield of polyester produced from .beta.-substituted-.beta.-propiolactones when these initiators are used is generally in the range of 20-60% crude yield after 7-14 days with the product having a molecular weight distribution or polydispersity, weight-average molecular weight (Mw) divided by number-average molecular weight (Mn) of around 15 (see Gross, R. A.; Zhang, Y.; Konrad, G.; Lenz, R. W. MACROMOLECULES 1988, 21, 2657-2668). The use of aluminum porphyrin initiators (see Aida, T.; Maekawa, S.A. and Inoue, S. in MACROMOLECULES 1988, 21, 1195-1202) will produce poly(.beta.-butyrolactone) from .beta.-butyrolactone with essentially quantitative yields and very narrow polydispersity (1.08-1.16), but the highest reported molecular weights of the polyester are less than 10,000 and the reaction times required are 5 to 20 days.
The use of distannoxane initiators (see Hori, Y.; Suzuki, M.; Takahashi, Y.; Yamaguchi, A.; Nishishita, T. in MACROMOLECULES 1993, 26, 5533-5534) is useful for the polymerization of racemic .beta.-butyrolactone, [S]-.beta.-butyrolactone, or [R]-.beta.-butyrolactone to high molecular weight polymer (Mw&gt;100,000) near quantitative yield in 4 h at 100.degree. C. The polydispersity of such produced poly(.beta.-butyrolactone) is typically 1.7-2.7. Syndiotactic poly(.beta.-butyrolactone) is produced when tributyltin methoxide is used to initiate the polymerization of racemic .beta.-butyrolactone (see Kemnitzer, J. E.; McCarthy, S. P.; Gross, R. A. in MACROMOLECULES 1993, 26, 1221-1229). The yields of polyester are 24-69% after 13-18 days and the molecular weights below 10,000 with the polydispersities are typically 1.04-1.12.
Polymerization of .beta.-substituted-l.beta.-propiolactones with dialkylzinc-water initiators require 5-7 days (see Zhang, Y.; Gross, R. A.; and Lenz, R.W. in MACROMOLECULES 1990, 23, 3206-32-12 and Tanahashi, N.; Doi, Y. in MACROMOLECULES 1991, 24, 5732-5733). Yields of polymer are generally 57-84%, but can reach 100% with racemic .beta.-butyrolactone. The polyester molecular weights are reported up to 50,000 with polydispersities typically 1.1-1.5. Unfortunately, the nature of the initiator is not well defined and the actual monomer-initiator ratio used is hard to quantitate.
The use of diethylzinc-[R]-(-)-3,3-dimethyl-1,2-butanediol to initiate the polymerization of racemic .beta.-butyrolactone in bulk leads to polyester with enhanced isotacticity (see Le Borgne, A. and Spassky, N. in POLYMER 1989, 30, 2312-2319). The reaction times are short, 2.5-15 h, and the yield can be as high as 84%. However, the molecular weight of the product, especially for the higher yield reaction (&lt;3,000), is very low.
Ring-opening polymerization of lactones other than .beta.-propiolactones with alkylzinc alkoxide initiators are known. The polymerization of .epsilon.-caprolactone and lactides are reported in the literature (see Barakat, I.; DuBois, Ph,; Jerome, R.; and Teyssie, Ph. in MACROMOLECULES 1991, 24, 6542-6545). Specifically, the use of ethylzinc isopropoxide is reported to copolymerize s-caprolactone and L,L-lactide (see Bero, M.; Kasperczyk, J.; and Adamus, G. in Makromol Chem. 1993, 194, 907-912 and 913-925). Molecular weights of these copolyesters are over 100,000 with polydispersities of 1.4, and the yields range from 60-89%. These polymerizations require 2.5-21 days at temperatures between 50 and 100.degree. C. Also, these investigators found that above 50.degree. C. the zinc initiators also are effective transesterification catalysts.
The use of a chiral diol-ethylzinc initiator has been disclosed for the polymerization of racemic .beta.-butyrolactone as described above (Le Borgne, A. and Spassky, N. in POLYMER 1989, 30, 2312-2319).
The previous attempts to polymerize .beta.-substituted-.beta.-propiolactones in the presence of various initiators have generally resulted in less than optimum product yields and length of reaction times.
Based on the foregoing, there is a need to provide a method of polymerizing .beta.-substituted-.beta.-propiolactones in the presence of an initiator which results in an improved yield of the desired polymer and/or an improved reaction time.